Search form

Main menu

You are here

Gadget Freak Case #275: A French Revolutionary Clock

If you run a search online, you will discover so-called metric or decimal clocks for sale. But be careful -- these clocks have what looks like a metric face (10 hour markings and numbering), but use a standard 12-hour mechanism, with 60 seconds per hour and 60 seconds per minute. Such a clock is not a metric clock at all. I have not been able to find a true metric clock for sale, other than antiques. If you find a modern one, please let me know.

I was asked to build an analog metric clock with hour, minute, and second hands that would run on Great Britain mains power. I have built clocks using 1 RPM synchronous motors, which run in step with 60 Hz 120v power. My first thought was to use a 50 Hz synchronous motor. But doing the math reveals this to be problematic. There are 60*60*24 or 86,400 seconds per in standard time, but 10*100*100 = 100,000 seconds per day in metric time. So a metric second is .864 of a standard second. With 100 metric seconds per revolution of the seconds hands, that makes 86.4 standard seconds per revolution, 60/86.4 = .694444444 RPMs.

Not easy to find a synchronous motor like that.

What kind of motor could operate accurately at an unusual speed? I decided a stepper motor would work. My first thought along this line was to use one motor at the proper RPM for the seconds hand, and gears to drive the minutes and hours hands. But a divide by 100 gear train would require four sets of wheels and pinions to be compact. I decided to use a separate stepper motor for each hand.